1. Field of the Invention
The present invention relates to a reflex coupler with an integrated organic light emitter, and particularly to a monolithically integrated CMOS reflex coupler with OLED light source.
2. Description of the Related Art
Light barriers have become widespread as contactless technical means for acquiring status, geometry, position or state information. Among these are industrial applications, such as in the automobile industry, consumer electronics, medical and metrological technology. One embodiment of the light barriers is reflex light barriers in which transmitter and receiver are not arranged in opposite, but adjacent manner. Such a combination suggests an integrated arrangement of transmitter and receiver as closely as possible, monolithically on a common substrate, if possible, with their field of view facing in the same direction, if possible.
FIG. 9 shows a principle construction of a monolithically integrated reflex light barrier. Both a receiver 900 and a transmitter 905 are integrated in a substrate 910. In operation, the transmitter 905 emits a light signal 940, which is reflected from an object 950 and then detected by the receiver 900. As opposed to the conventional light barrier, a signal is only generated in an absence of the object 950 and/or upon reflection by the object 950.
Conventional integrated reflex light barriers are based on a CMOS (complementary metal oxide semiconductor) reception and evaluation chip, as well as an emitter of conventional (inorganic) light-emitting diodes. Both technologies utilize materials and processes different from each other. The CMOS technology mostly is based on monocrystalline silicon, while conventional light-emitting diodes mostly utilize monocrystalline III-V semiconductors. Thus, corresponding devices are not monolithic, but integratable with each other exclusively in hybrid manner.
Reflex couplers work according to the same principle as the reflex light barriers, i.e. the transmitter 905 and the receiver 900 are optically coupled to each other via a reflection of the light signal 940. In absence of the reflection, no coupling between the transmitter 905 and the receiver 900 is present. Reflex couplers thus may also serve as switches, i.e. electrical signals are passed on from one device to another device in absence of the reflection, wherein at the same time a galvanic separation of circuits is realized.
As light transmitters 905 in a reflex coupler, often light-emitting diodes (LEDs) are used, which emit infrared light or red light, and photodiodes, phototransistors, photothyristors, phototriacs, Schmitt phototriggers and Darlington phototransistors, for example, are used as light receiver or photodetector 900, i.e. the light receiver 900 generally comprises one or more pn junctions. The light transmitter 905 and the light receiver 900 are electrically insulated from each other. What is transmitted is continuous or alternating light, and the reflected light is assessed with respect to, maybe time-dependent, intensity, frequency, phase or wavelength.
Photodiodes as potential photodetectors 900 can be implemented in a standard CMOS process at various pn interfaces, and FIG. 10 shows an example implemented in a known n-well CMOS process. Here, a n-doped well (n well) 920 is formed in a p-doped substrate (p substrate) 910, said well comprising a p+-doped layer 930 on the side facing away from the p substrate 910. As a final layer, the p substrate 910 comprises an oxide layer 940, and an ILD (inter-layer dielectric) layer 950, followed by an IMD (inter-metal dielectric) layer 960, is deposited. The oxide layer 940, the ILD layer 950 and the IMD layer 960, for example, comprise a dielectric material and are translucent. Various pn junctions are characterized by diodes 962, 964 and 975.
Incident light beams 990 create a charge carrier pair 985 of opposite polarity in the n well 920, which is separated according to the polarity and generates an electrical signal. The photodetector 900 thus is formed by the p substrate 910, the n well 920, the p+-doped layer 930, as well as by the oxide layer 940. Necessary contacts for sensing the photodetector signal are not shown in FIG. 10 for reasons of clarity. FIG. 10 also shows a further photodiode 975, which is formed of a pn junction from the p substrate 910 and an n+-doped surface layer 970. The light signals 980 represent reflected light at the surface layer 970.
Similar to reflex light barriers, fully integrated conventional reflex couplers are based on a CMOS reception chip as photodetector 900 and a CMOS evaluation chip as well as an emitter 905 of conventional (inorganic) light-emitting diodes. In conventional reflex couplers, both technologies also use materials and processes different from each other (CMOS: mostly silicon, LED: mostly III-V semiconductors) and thus are not monolithic, but only integratable with each other in hybrid manner.
Conventional light-emitting diodes of inorganic semiconductors, such as GaAs and related III-V semiconductors, have been known for decades. A basic principle of such light-emitting diodes is that, by applying an electrical voltage, electrons and holes are injected in a semiconductor and combine in radiating manner in a recombination zone under light emission. Nevertheless, light-emitting diodes on the basis of inorganic semiconductors also have significant disadvantages for many applications. A substantial disadvantage is, as already mentioned, that they are mostly applied only to III-V semiconductor backgrounds.
As an alternative to inorganic light-emitting diodes, light-emitting diodes on the basis of organic semiconductors have made great progress in the last few years. For example, organic electroluminescence is presently getting much attention as a medium suited for displays. Organic light-emitting diodes comprise an organic layer sequence with a thickness of typically around 100 nm, which is inserted between an anode and a cathode. Often, glass is used as a substrate, onto which a transparent, electrically conducting oxide is applied, such as indium tin oxide (ITO). Thereupon follows the organic layer sequence, which comprises hole-transporting material, emitting material and electron-transporting material. Then, mostly a metallic cathode follows.
In general, it is distinguished between organic light-emitting diode (OLEDs) as top emitters and OLEDs as bottom emitters. Typically, bottom emitters mainly emit the light signal 950 through the substrate, whereas top emitters emit in a direction away from the substrate.
FIG. 11 shows an organic light-emitting diode (OLED) 905, which is formed as a top emitter. Here, an electrode 925, an organic layer sequence 935 and a transparent electrode 945 are applied on a substrate 915. The contacting is done via a terminal 955 to the electrode 925, as well as via a terminal 965 to the transparent electrode 945. The substrate 915 mostly comprises non-transparent material and the electrode 925 a metal, for example. This results in the fact that, when applying a corresponding voltage at the terminal 955 and 965, a light signal 940 generated in the organic layer sequence 935 is emitted upward through the transparent electrode 945 (for example of ITO) in the type of illustration shown.
The light signal 940 in FIG. 11 indicates a main emission direction. Light generated in the organic layer sequence 935 does, however, also propagate along the organic layer sequence 935 or along the transparent electrode 945 and is also partially emitted laterally, as far as no lateral shielding is present.
Reflex couplers with inorganic emitter 905 (and detector 900) already are known. Organic light-emitting diode displays combined with an optical proximity switch and based on an organic emitter are already known. In DE 10244452 B4, such an optoelectronic switch used for a touch-sensitive (OLED) display is described.
As stated, since conventional LEDs predominantly use III-V semiconductors, and the detector circuit (i.e. the photodetector 900 and control circuit) is mostly based on silicon, both devices cannot be produced on the same substrate, and integration hence proves difficult. A possible hybrid integration in reflex couplers, such as it is known, in principle, necessitates a greater fabrication effort and does not allow for general price regression, especially in high numbers of pieces. Furthermore, due to the hybrid manner of construction, the reliability necessary for automobile applications only can be achieved at extremely high costs.